Due to popular consumer demand, there is a trend to develop smaller mobile terminals, such as cell phones, which makes more and more mobile terminals use internal antennas. On one hand, the bandwidth of regular internal antennas for mobile terminals (in particular antennas working at low frequency bands of 850 MHz/900 MHz) is dependent on the antennas and clearance, and the motherboard length (i.e. the main ground length of antennas) is also an important factor determining the antenna bandwidth. On the other hand, fierce market competition makes customer experience increasingly important, while the antenna performance of a mobile terminal directly determines a customer's experience. At the same time, there are more and more high-speed data services on mobile networks, all of which require that mobile terminals have excellent antenna performance. That is because, under the same wireless network environment, mobile terminals with better antenna performance can use modulation methods with higher speed, which can optimize customer experience and moreover, can improve network utilization rate and save resources. Therefore, the design of high performance and wide bandwidth antennas inside small mobile terminals has become a hot research topic in the industry. Particularly in the current antenna design for mobile terminals, low frequency band antennas require motherboard lengths to exceed 100 mm; otherwise it would be difficult to achieve sufficient bandwidth. In practical development, however, some small terminals are unable to meet the above condition, which makes it difficult to develop antennas. For example, FIG. 1 shows a PIFA antenna emulation on a motherboard with the size of 62 mm*100 mm*1 mm (length*width*height), the antenna has a height of 9 mm and is a GSM850/PCS dual frequency antenna. The emulation performance of the antenna is shown in FIG. 1. From the emulation result, the antenna can substantially meet performance requirements at frequency bands of GSM850 and PCS. If the size of this motherboard is revised to 62 mm*62 mm*1 mm (to meet miniaturization requirements for mobile terminals) and the antenna size is adjusted and optimized, the new antenna structure and emulation result are shown in FIG. 2. It can be seen from the emulation result in FIG. 2 that in frequency bands of the high frequency PCS, the impact of motherboard length on the antenna's performance is relatively small and the antenna's performance can substantially meet requirements. In the frequency bands of the low frequency GSM850, however, the antenna bandwidth deteriorates rapidly, which is unable to meet the requirement for radio-frequency performance. Therefore, the miniaturization of mobile terminals results in significant negative impacts on the antenna's bandwidth performance. As a result, it is indeed necessary to provide a technology for strengthening the performance of internal antennas of small mobile terminals.